PROJECT SUMMARY Hexanucleotide repeat expansion in a non-coding region of C9orf72 is the most common genetic cause of both amyotrophic lateral sclerosis (ALS) and frontal temporal dementia (FTD). One potential pathogenic mechanism is the aberrant accumulation of dipeptide repeat (DPR) proteins produced by repeat-associated non-AUG (RAN) translation in all six reading frames (poly-GA, poly-GR, poly-PA, poly-PR and poly-PG) of both sense and antisense RNAs. Abnormal cytoplasmic inclusions of these DPR proteins have been found in C9ORF72 patient tissues and cells. Several lines of evidence indicate that some forms of the DPR proteins can be pathogenic, yet little is known how the DPRs are generated from the expanded repeats. The goal of this proposal is to understand the molecular mechanisms and identify genetic modifiers of DPR production from both sense and antisense repeats, combining the genome-scale CRISPR/Cas9 knockout screening technology with biochemical and molecular approaches. We will primarily focus on two major steps of repeat RNA processing that could influence the final protein expression level: RNA nuclear export and RAN translation. We will dissect the molecular pathways of nuclear export for both sense and antisense repeats, what factors mediate this process, and how this is affected by C9-mediated toxicity on nucleocytoplasmic transport. We will decipher the molecular mechanisms of C9 RAN translation, and examine whether there are common modifiers between sense and antisense repeats. Altogether, these studies can provide novel insights on how DPRs are produced from C9 repeats and what approaches can be taken to prevent it. This will guide the development of research tools to understand how DRPs contribute to the disease pathogenesis. It will also possibly provide potential therapeutic targets to reduce DPR-mediated toxicity by inhibiting their production.